Characterization of membrane permeability alterations induced in Vero cells by Clostridium perfringens enterotoxin

Innovative and Highly Sensitive Detection of Clostridium perfringens Enterotoxin Based on Receptor Interaction and Monoclonal Antibodies

Clostridium perfringens enterotoxin (CPE) regularly causes food poisoning and antibiotic-associated diarrhea; therefore, reliable toxin detection is crucial. To this aim, we explored stationary and mobile strategies to detect CPE either exclusively by monoclonal antibodies (mAbs) or, alternatively, by toxin-enrichment via the cellular receptor of CPE, claudin-4, and mAb detection. Among the newly generated mAbs, we identified nine CPE-specific mAbs targeting five distinct epitopes, among them mAbs recognizing CPE bound to claudin-4 or neutralizing CPE activity in vitro. In surface plasmon resonance experiments, all mAbs and claudin-4 revealed excellent affinities towards CPE, ranging from 0.05 to 2.3 nM. Integrated into sandwich enzyme-linked immunosorbent assays (ELISAs), the most sensitive mAb/mAb and claudin-4/mAb combinations achieved similar detection limits of 0.3 pg/mL and 1.0 pg/mL, respectively, specifically detecting recombinant CPE from spiked feces and native CPE from 30 different C. perfringens culture supernatants. The implementation of mAb- and receptor-based ELISAs into a mobile detection platform enabled the fast detection of CPE, which will be helpful in clinical laboratories to diagnose diarrhea of assumed bacterial origin. In conclusion, we successfully employed an endogenous receptor and novel high affinity mAbs for highly sensitive and specific CPE-detection. These tools will be useful for both basic and applied research.

Clostridium perfringens Enterotoxin: Action, Genetics, and Translational Applications

Clostridium perfringens enterotoxin (CPE) is responsible for causing the gastrointestinal symptoms of several C. perfringens food- and nonfood-borne human gastrointestinal diseases. The enterotoxin gene (cpe) is located on either the chromosome (for most C. perfringens type A food poisoning strains) or large conjugative plasmids (for the remaining type A food poisoning and most, if not all, other CPE-producing strains). In all CPE-positive strains, the cpe gene is strongly associated with insertion sequences that may help to assist its mobilization and spread. During disease, CPE is produced when C. perfringens sporulates in the intestines, a process involving several sporulation-specific alternative sigma factors. The action of CPE starts with its binding to claudin receptors to form a small complex; those small complexes then oligomerize to create a hexameric prepore on the membrane surface. Beta hairpin loops from the CPE molecules in the prepore assemble into a beta barrel that inserts into the membrane to form an active pore that enhances calcium influx, causing cell death. This cell death results in intestinal damage that causes fluid and electrolyte loss. CPE is now being explored for translational applications including cancer therapy/diagnosis, drug delivery, and vaccination.

A synthetic peptide corresponding to the extracellular loop 2 region of claudin-4 protects against Clostridium perfringens enterotoxin in vitro and in vivo

Clostridium perfringens enterotoxin (CPE) action starts when the toxin binds to claudin receptors. Claudins contain two extracellular loop domains, with the second loop (ECL-2) being slightly smaller than the first. CPE has been shown to bind to ECL-2 in receptor claudins. We recently demonstrated that Caco-2 cells (a naturally CPE-sensitive enterocyte-like cell line) can be protected from CPE-induced cytotoxicity by preincubating the enterotoxin with soluble full-length recombinant claudin-4 (rclaudin-4), which is a CPE receptor, but not with recombinant nonreceptor claudins, such as rclaudin-1. The current study evaluated whether a synthetic peptide corresponding to the claudin-4 ECL-2 sequence can similarly inhibit CPE action in vitro and in vivo. Significant protection of Caco-2 cells was also observed using either rclaudin-4 or the claudin-4 ECL-2 peptide in both a preincubation assay and a coincubation assay. This inhibitory effect was specific, since rclaudin-1 and a synthetic peptide based on the claudin-1 ECL-2 offered no protection to Caco-2 cells. However, the claudin-4 ECL-2 peptide was unable to neutralize cytotoxicity if CPE had already bound to Caco-2 cells. When the study was repeated in vivo using a rabbit small intestinal loop assay, preincubation or coincubation of CPE with the claudin-4 ECL-2 peptide significantly and specifically inhibited the development of CPE-induced luminal fluid accumulation and histologic lesions in rabbit small intestinal loops. No similar in vivo protection from CPE was afforded by the claudin-1 ECL-2 peptide. These results suggest that claudin-4 ECL-2 peptides should be further investigated for their potential therapeutic application against CPE-associated disease.

Human claudin-8 and -14 are receptors capable of conveying the cytotoxic effects of Clostridium perfringens enterotoxin

Clostridium perfringens enterotoxin (CPE) contributes to several important human gastrointestinal (GI) diseases. This toxin and its derivatives are also being explored for translational applications, i.e., cancer therapy or drug delivery. Some, but not all, members of the 24-member claudin (Cldn) family of mammalian tight junction proteins can serve as CPE receptors. Among the human Cldns (hCldns), hCldn-3 and -4 are known to convey CPE sensitivity when expressed by fibroblast transfectants. However, other Cldns are also reportedly expressed in the intestines, where they might contribute to natural CPE-mediated GI disease, and in other organs, where they might react with CPE-based therapeutics. Therefore, the current study assessed whether two additional hCldns beside hCldn-3 and -4 are also functional CPE receptors. Using Cldn-expressing transfectants, hCldn-8 and -14 were shown to convey CPE-mediated cytotoxicity at pathophysiologically relevant concentrations of this toxin, although ~2-to-10-fold less efficiently than hCldn-4. Site-directed mutagenesis then demonstrated that the N¹⁴⁶ residue in hCldn-14 and the S¹⁵¹ residue in hCldn-8 are largely responsible for modulating the weaker CPE binding properties of hCldn-8 and -14 versus hCldn-4, which broadens understanding of Cldn:CPE binding interactions. Since Cldn-8 and -14 are reportedly expressed in mammalian intestines, the current results support the possibility that these two hCldns contribute to natural CPE-mediated gastrointestinal disease and could be CPE-based therapeutic targets for cancers overexpressing those claudins. However, these results also suggest caution during therapeutic use of CPE, which might trigger toxic side effects in normal human tissues producing hCldn-8 or -14, as well as in those producing hCldn-3 or -4.

IMPORTANCE Clostridium perfringens enterotoxin (CPE) is responsible for the gastrointestinal symptoms of the second-most-common bacterial food-borne illness and is also being explored for use as a cancer therapeutic or for increasing drug delivery. Until now, the only known human CPE receptors were claudin-3 and -4. This work shows that human claudin-8 and -14 can also bind CPE and convey cytotoxicity, although slightly less efficiently than claudin-3 and -4. The claudin-8 and -14 residues responsible for this weaker CPE binding were identified, shedding new light on CPE:claudin interactions.

Clostridium perfringens enterotoxin (CPE) is responsible for the gastrointestinal symptoms of the second-most-common bacterial food-borne illness and is also being explored for use as a cancer therapeutic or for increasing drug delivery. Until now, the only known human CPE receptors were claudin-3 and -4. This work shows that human claudin-8 and -14 can also bind CPE and convey cytotoxicity, although slightly less efficiently than claudin-3 and -4. The claudin-8 and -14 residues responsible for this weaker CPE binding were identified, shedding new light on CPE:claudin interactions.

Compositional and stoichiometric analysis of Clostridium perfringens enterotoxin complexes in Caco-2 cells and claudin 4 fibroblast transfectants

Clostridium perfringens enterotoxin (CPE) binds to host cell receptors, forming a small complex precursor for two large complexes reportedly having molecular masses of approximately 155 or approximately 200 kDa. Formation of the approximately 155 kDa complex causes a Ca(2+) influx that leads to apoptosis or oncosis. CPE complex composition is currently poorly understood, although occludin was identified in the approximately 200 kDa complex. The current study used heteromer gel shift analysis to show both CPE large complexes contain six CPE molecules. Ferguson plots and size exclusion chromatography re-sized the approximately 155 and approximately 200 kDa complexes as approximately 425-500 kDa and approximately 550-660 kDa respectively. Co-immunoprecipitation and electroelution studies demonstrated both CPE-binding and non-CPE-binding claudins are associated with all three CPE complexes in Caco-2 cells and with small complex and approximately 425-500 kDa complex of claudin 4 transfectants. Fibroblast transfectants expressing claudin 4 or C-terminal truncated claudin 4 were CPE-sensitive and formed the approximately 425 kDa complex, indicating claudin-induced cell signalling is not required for CPE action and that expression of a single receptor claudin suffices for approximately 425-500 kDa CPE complex formation. These results identify CPE as a unique toxin that combines with tight junction proteins to form high-molecular-mass hexameric pores and alter membrane permeability.

The importance of calcium influx, calpain and calmodulin for the activation of CaCo-2 cell death pathways by Clostridium perfringens enterotoxin

CaCo-2 cells exhibit apoptosis when treated with low doses of Clostridium perfringens enterotoxin (CPE), but develop oncosis when treated with high CPE doses. This study reports that the presence of extracellular Ca(2+) in treatment buffers is important for normal activation of both those cell death pathways in CPE-treated CaCo-2 cells. Normal development of CPE-induced cell death pathway effects, such as morphologic damage, DNA fragmentation, caspase activation, mitochondrial membrane depolarization and cytochrome c release, was strongly inhibited when CaCo-2 cells were CPE-treated in Ca(2+)-free buffers. When treatment buffers contained Ca(2+), CPE caused a rapid increase in CaCo-2 cell Ca(2+) levels, apparently because of increased Ca(2+) influx through a CPE pore. High CPE doses caused massive changes in cellular Ca(2+) levels that appear responsible for activating oncosis, whereas low CPE doses caused less perturbations in cellular Ca(2+) levels that appear responsible for activating apoptosis. Both CPE-induced apoptosis and oncosis were found to be calmodulin- and calpain-dependent processes. As Ca(2+) levels present in the intestinal lumen resemble those of Ca(2+)-containing treatment buffers used in this study, perturbations in cellular Ca(2+) levels and calpain/calmodulin-dependent processes are also probably important for inducing enterocyte cell death during CPE-mediated gastrointestinal disease.

Fine mapping of the N-terminal cytotoxicity region of Clostridium perfringens enterotoxin by site-directed mutagenesis

Clostridium perfringens enterotoxin (CPE) has a unique mechanism of action that results in the formation of large, sodium dodecyl sulfate-resistant complexes involving tight junction proteins; those complexes then induce plasma membrane permeability alterations in host intestinal epithelial cells, leading to cell death and epithelial desquamation. Previous deletion and point mutational studies mapped CPE receptor binding activity to the toxin's extreme C terminus. Those earlier analyses also determined that an N-terminal CPE region between residues D45 and G53 is required for large complex formation and cytotoxicity. To more finely map this N-terminal cytotoxicity region, site-directed mutagenesis was performed with recombinant CPE (rCPE). Alanine-scanning mutagenesis produced one rCPE variant, D48A, that failed to form large complexes or induce cytotoxicity, despite having normal ability to bind and form the small complex. Two saturation variants, D48E and D48N, also had a phenotype resembling that of the D48A variant, indicating that both size and charge are important at CPE residue 48. Another alanine substitution rCPE variant, I51A, was highly attenuated for large complex formation and cytotoxicity, but rCPE saturation variants I51L and I51V displayed a normal large complex formation and cytotoxicity phenotype. Collectively, these mutagenesis results identify a core CPE sequence extending from residues G47 to I51 that directly participates in large complex formation and cytotoxicity.

Death pathways activated in CaCo-2 cells by Clostridium perfringens enterotoxin

Clostridium perfringens enterotoxin (CPE), a 35-kDa polypeptide, induces cytotoxic effects in the enterocyte-like CaCo-2 cell culture model. To identify the mammalian cell death pathway(s) mediating CPE-induced cell death, CaCo-2 cultures were treated with either 1 or 10 micro g of CPE per ml. Both CPE doses were found to induce morphological damage and DNA cleavage in CaCo-2 cells. The oncosis inhibitor glycine, but not a broad-spectrum caspase inhibitor, was able to transiently block both of those pathological effects in CaCo-2 cells treated with the higher, but not the lower, CPE dose. Conversely, a caspase 3/7 inhibitor (but not glycine or a caspase 1 inhibitor) blocked morphological damage and DNA cleavage in CaCo-2 cells treated with the lower, but not the higher, CPE dose. Collectively, these results indicate that lower CPE doses cause caspase 3/7-dependent apoptosis, while higher CPE doses induce oncosis. Apoptosis caused by the lower CPE dose was shown to proceed via a classical pathway involving mitochondrial membrane depolarization and cytochrome c release. As the CPE concentrations used in this study for demonstrating apoptosis and oncosis have pathophysiologic relevance, these results suggest that both oncosis and apoptosis may occur in the intestines during CPE-associated gastrointestinal disease.

The complex interactions between Clostridium perfringens enterotoxin and epithelial tight junctions

Clostridium perfringens enterotoxin (CPE) is responsible for the diarrheal symptoms of C. perfringens type A food poisoning and antibiotic-associated diarrhea. The CPE protein consists of a single 35 kDa polypeptide with a C-terminal receptor-binding region and an N-terminal toxicity domain. Under appropriate conditions, CPE can interact with structural components of the epithelial tight junctions, including certain claudins and occludin. Those interactions can affect tight junction structure and function, thereby altering paracellular permeability and (possibly) contributing to CPE-induced diarrhea. However, the tight junction effects of CPE require cellular damage as a prerequisite. CPE induces cellular damage via its cytotoxic activity, which results from plasma membrane permeability alterations caused by formation of a approximately 155 kDa CPE-containing complex that may correspond to a pore. Thus, CPE appears to be a bifunctional toxin that first induces plasma membrane permeability alterations; using the resultant cell damage, CPE then gains access to tight junction proteins and affects tight junction structure and function.

CaCo-2 cells treated with Clostridium perfringens enterotoxin form multiple large complex species, one of which contains the tight junction protein occludin

The previous model for the action of Clostridium perfringens enterotoxin (CPE) proposed that (i) CPE binds to host cell receptor(s), forming a small ( approximately 90 kDa) complex, (ii) the small complex interacts with other eucaryotic protein(s), forming a large ( approximately 160 kDa) complex, and (iii) the large complex triggers massive permeability changes, thereby inducing enterocyte death. In the current study, Western immunoblot analysis demonstrated that CPE bound to CaCo-2 human intestinal cells at 37 degrees C forms multiple large complex species, with apparent sizes of approximately 200, approximately 155, and approximately 135 kDa. These immunoblot experiments also revealed that occludin, an approximately 65-kDa tight junction protein, is present in the approximately 200-kDa large complex but absent from the other large complex species. Immunoprecipitation studies confirmed that occludin physically associates with CPE in large complex material and also indicated that occludin is absent from small complex. These results strongly suggest that occludin becomes associated with CPE during formation of the approximately 200-kDa large complex. A postbinding association between CPE and occludin is consistent with the failure of rat fibroblast transfectants expressing occludin to bind CPE in the current study. Those occludin transfectants were also insensitive to CPE, strongly suggesting that occludin expression is not sufficient to confer CPE sensitivity. However, the occludin-containing, approximately 200-kDa large complex may contribute to CPE-induced cytotoxicity, because nontoxic CPE point mutants did not form any large complex species. By showing that large complex material is comprised of several species (one containing occludin), the current studies indicate that CPE action is more complicated than previously appreciated and also provide additional evidence for CPE interactions with tight junction proteins, which could be important for CPE-induced pathophysiology.

Identification of a Clostridium perfringens enterotoxin region required for large complex formation and cytotoxicity by random mutagenesis

Clostridium perfringens enterotoxin (CPE), a single polypeptide of 319 amino acids, has a unique multistep mechanism of action. In the first step, CPE binds to claudin proteins and/or a 50-kDa eukaryotic membrane protein receptor, forming a small ( approximately 90-kDa) complex. This small complex apparently then associates with a 70-kDa eukaryotic membrane protein, resulting in formation of a large complex that induces the onset of membrane permeability alterations. To better define the boundaries of CPE functional regions and to identify specific amino acid residues involved in various steps of CPE action, in this study we subjected the cloned cpe gene to random mutagenesis in XL-1 Red strains of Escherichia coli. Seven CPE random mutants with reduced cytotoxicity for Vero cells were phenotypically characterized for the ability to complete each step in CPE action. Five of these seven recombinant CPE (rCPE) random mutants (G49D, S59L, R116S, R137G, and S167P) exhibited binding characteristics similar to those of rCPE or native CPE, while the Y310C and W226Stop mutants showed reduced binding and no binding, respectively, to brush border membranes. Interestingly, two completely nontoxic mutants (G49D and S59L) were able to bind and form small complex but they did not mediate any detectable large complex formation. Another strongly attenuated mutant, R116S, formed reduced amounts of an anomalously migrating large complex. Collectively, these results provide further support for large complex formation being an essential step in CPE action and also identify the CPE region ranging from residues approximately 45 to 116 as important for large complex formation. Finally, we also report that limited removal of extreme N-terminal CPE sequences, which may occur in vivo during disease, stimulates cytotoxic activity by enhancing large complex formation.

Deletion analysis of the Clostridium perfringens enterotoxin

To further our knowledge of the structure-function relationship and mechanism of action of the Clostridium perfringens enterotoxin (CPE), a series of recombinant CPE (rCPE) species containing N- and C-terminal CPE deletion fragments was constructed by recombinant DNA approaches. Each rCPE species was characterized for its ability to complete the first four early steps in the action of CPE, putatively ordered as specific binding, a postbinding physical change to bound CPE, large-complex formation, and induction of alterations in small-molecule membrane permeability. These studies demonstrated that (i) at least 44 amino acids can be removed from the N terminus of CPE without loss of cytotoxicity, (ii) removal of the first 53 amino acids from the N terminus of CPE produces a fragment that appears to be noncytotoxic because it cannot undergo the post-binding physical change step in CPE action, (iii) removal of as few as five amino acids from the C terminus of CPE produces a noncytotoxic fragment lacking receptor binding activity, and (iv) a fragment lacking the first 44 N-terminal amino acids of native CPE formed twice as much large complex and was twice as cytotoxic as native CPE. From these structure-function results, it appears that the minimum-size cytotoxic CPE fragment comprises approximately residues 45 to 319 of native CPE. Results from these deletion fragment studies have also contributed to our understanding of CPE action by (i) independently supporting previous suggestions that binding, the postbinding physical change step, and large-complex formation represent important steps in CPE cytotoxicity and (ii) providing independent evidence confirming the putative sequential order of these early events in CPE action.

Mitogenic activity of Clostridium perfringens enterotoxin in human peripheral lymphocytes

Clostridium perfringens enterotoxin (CPE) was found to possess interferon (IFN)-producing and mitogenic activities to human peripheral blood mononuclear cells. Both activities were demonstrated only in the T lymphocyte-rich fraction from healthy volunteers. The IFN produced appeared to be gamma-type since the activity of the IFN was neutralized by antiserum against human IFN-gamma. With formalin-treated CPE, the IFN-producing and mitogenic activities were weakly found. Similar findings were also obtained in the mouse lethality and cytotoxicity to Vero (African green monkey) cells, suggesting that the biological activities of the CPE molecule may be existing on the similar (or the same) sites. From these findings, human peripheral T cells may be one of useful reagents to study the mode of action of CPE since CPE was found to be a T cell mitogen which is supposed to be a superantigen.

A conjugated synthetic peptide corresponding to the C-terminal region of Clostridium perfringens type A enterotoxin elicits an enterotoxin-neutralizing antibody response in mice

A synthetic peptide homolog corresponding to the C-terminal 30 amino acids of Clostridium perfringens type A enterotoxin (CPE) was conjugated to a thyroglobulin carrier and used to immunize mice. Conjugate-immunized mice produced antibodies which neutralized native CPE cytotoxicity, at least in part, by blocking enterotoxin binding. This peptide may be useful for the development of a vaccine to protect against CPE-mediated disease.

Mapping of functional regions of Clostridium perfringens type A enterotoxin

Studies were conducted to allow construction of an initial map of the structure-versus-function relationship of the Clostridium perfringens type A enterotoxin (CPE). Removal of the N-terminal 25 amino acids of CPE increased the primary cytotoxic effect of CPE but did not affect binding. CPE sequences required for at least four epitopes were also identified.

Studies of Clostridium perfringens enterotoxin action at different temperatures demonstrate a correlation between complex formation and cytotoxicity

The cytotoxicity of Clostridium perfringens enterotoxin (CPE) was completely blocked in Vero cells continuously CPE treated at 4 degrees C. [125I]CPE-specific binding to either Vero cells or isolated rabbit intestinal brush border membranes (BBMs) was lower at 4 degrees C than at 24 or 37 degrees C, but reduced enterotoxin binding could not totally explain the loss of cytotoxicity at low temperature. Insertion of enterotoxin into Vero cell membranes or BBMs was temperature independent. However, CPE complex formation (A. P. Wnek and B. A. McClane, Infect. Immun. 57:574-581, 1989) in BBMs and Vero cells was blocked at 4 degrees C. When Vero cells were CPE treated at 4 degrees C, washed to remove unbound toxin, and then shifted to 37 degrees C, complex formation and cytotoxicity were rapidly detected. When CPE binding and complex formation were permitted for 2 min at 37 degrees C, and the Vero cells were then shifted to 4 degrees C, cytotoxicity was detectable at 4 degrees C. These results are consistent with complex formation, rather than complex activity, being the temperature-sensitive step in CPE action which is blocked at 4 degrees C. These studies demonstrate a strong correlation between complex formation and cytotoxicity and are consistent with complex involvement in CPE cytotoxicity. These studies also strongly suggest that CPE insertion precedes both complex formation and induction of small-molecule permeability changes.

Evidence that alterations in small molecule permeability are involved in the Clostridium perfringens type A enterotoxin-induced inhibition of macromolecular synthesis in Vero cells

The mechanism by which Clostridium perfringens enterotoxin (CPE) simultaneously inhibits RNA, DNA, and protein synthesis is unknown. In the current study the possible involvement of small molecule permeability alterations in CPE-induced inhibition of macromolecular synthesis was examined. Vero cells CPE-treated in minimal essential medium (MEM) completely ceased net precursor incorporation into RNA and protein within 15 minutes of CPE treatment. However, RNA and protein synthesis continued for at least 30 minutes in Vero cells CPE-treated in buffer (ICIB) approximating intracellular concentrations of most ions. Addition of intracellular concentrations of amino acids to ICIB (ICIB-AA) caused a further small but detectable increase in protein synthesis in CPE-treated cells. ICIB did not affect CPE-specific binding levels or rates. Similar small molecule permeability changes (i.e., 86Rb-release) were observed in cells CPE-treated in either ICIB or in Hanks' balanced salt solution. Collectively these findings suggest that CPE-treatment of cells in ICIB-AA ameliorates CPE-induced changes in intracellular concentrations of ions and amino acids and permits the continuation of RNA and protein synthesis. These results are consistent with and support the hypothesis that permeability alterations for small molecules are involved in the CPE-induced inhibition of precursor incorporation into macromolecules in Vero cells.

Characterization of calcium involvement in the Clostridium perfringens type A enterotoxin-induced release of 3H-nucleotides from Vero cells

This report characterizes the involvement of Ca2+ in the release of nucleotides from Vero cells caused by Clostridium perfringens enterotoxin (CPE). A positive linear correlation was observed between increased CPE-induced nucleotide-release and increased extracellular calcium over the range 0.01 to 10 mM calcium. Above 5 mM Ca2+, CPE-specific lysis (i.e. disintegration of cells as monitored by light microscopy) was observed. Addition of 1.7 mM Ca2+ to Vero cells previously CPE-treated in Ca2+-free buffer rapidly increased nucleotide-release, even when cells had been previously incubated for 1 h at 37 degrees C in Ca2+-free buffer. Withdrawal of Ca2+, even after the onset of nucleotide-release, halted further CPE-induced nucleotide-release. These results indicate that Ca2+ must be continuously present for significant CPE-induced nucleotide-release. However, withdrawal of Ca2+ did not reverse membrane bleb formation by CPE. This differentiates bleb formation and nucleotide-release (both Ca2+-dependent CPE effects) and suggests that nucleotide-release does not result simply from bleb formation. Lastly, it was shown that other ions besides physiologic Ca2+ (1.7 mM) are required for CPE-induced nucleotide-release. Interestingly, a role for other ions (but not physiologic Ca2+) is also shown for 86Rb-release by CPE (an early Ca2+-independent CPE effect). This indicates that extracellular ions other than physiologic Ca2+ can be required for both Ca2+-independent and Ca2+-dependent CPE effects.

The effects of Clostridium perfringens enterotoxin on intracellular levels or transport of uridine, thymidine and leucine do not fully explain enterotoxin-induced inhibition of macromolecular synthesis in Vero cells

Clostridium perfringens type A enterotoxin (CPE) has been shown previously to inhibit the incorporation of radiolabeled precursors into acid-insoluble material but the mechanism of inhibition is unknown. It has also been shown that extracellular calcium is required for some CPE effects. In this report, it is shown that CPE completely and virtually simultaneously inhibits incorporation of precursors into RNA, DNA and protein in either the presence or absence of extracellular divalent cations and that changes in intracellular precursor levels did not consistently correlate with this CPE-induced inhibition of incorporation. These results strongly suggest that CPE can inhibit macromolecular synthesis, not just inhibit precursor transport. It is inferred from this that CPE can affect DNA and RNA synthesis, and possibly protein synthesis, by altering other cellular processes besides, or in addition to, precursor transport and these effects then lead to a shutdown of macromolecular synthesis.

Evaluation of ELISA, RPLA, and Vero cell assays for detecting Clostridium perfringens enterotoxin in faecal specimens

Three hundred and ninety two faecal specimens from 70 separate outbreaks of suspected Clostridium perfringens food poisoning were examined by enzyme linked immunosorbent assay (ELISA), reversed passive latex agglutination (RPLA), and Vero cell assays for the presence of enterotoxin. Although the most time consuming method, ELISA was the most specific and reproducible. RPLA was slightly more sensitive than ELISA, but it showed some non-specific reactions. The Vero cell assay was the least sensitive and least reproducible method, being affected by some non-specific cytotoxic and cytotonic reactions. Normal rabbit serum should be included in the Vero cell assay as a control for the neutralisation of cytotoxic effects.

Isolation and function of a Clostridium perfringens enterotoxin fragment

A fragment was obtained by treating Clostridium perfringens enterotoxin with 2-nitro-5-thiocyanobenzoic acid, a reagent which specifically cleaves the amino-terminal peptide bond of cysteine residues. The fragment (molecular weight, 15,000) was purified by high-performance liquid chromatography. The fragment had no cytotoxic effect on Vero cells but competitively inhibited enterotoxin-induced 51Cr release. Binding of 125I-labeled fragment to Vero cells was comparable to that of enterotoxin. Moreover, 125I-labeled fragment did not bind to FL cells, which lack receptor for enterotoxin. We conclude that the fragment contains the binding domain of enterotoxin. The amino acid composition of the fragment suggests that it is located on the carboxyl-terminal part of enterotoxin.

Interferon pretreatment enhances the sensitivity of Vero cells to Clostridium perfringens type A enterotoxin

Treatment of Vero (African green monkey kidney) cells with interferon (IFN) before the addition of Clostridium perfringens type A enterotoxin (CPE) significantly increased the sensitivity of these cells to CPE. IFN pretreatment caused a subsequent two- to four-fold increase in CPE-induced membrane permeability alterations and also decreased the time of CPE treatment required before the onset of permeability alterations and morphologic damage. Enhancement of CPE activity was dependent on the amount of IFN added during pretreatment and on the duration of IFN pretreatment incubations. Potentiation of CPE activity was observed following pretreatment of Vero cells with natural human IFN-alpha or IFN-gamma or Roferon recombinant human IFN-alpha. However, pretreatment with mouse IFN did not affect CPE activity. IFN pretreatment did not grossly enlarge the size of the functional hole produced in plasma membranes by CPE. IFN pretreatment of Vero cells slightly increased CPE specific binding, but this effect occurred kinetically after the enhancement of CPE toxicity. These results suggest that IFN pretreatment enhances the action of CPE on IFN pretreated Vero cells by increasing the sensitivity of these cells to the action of CPE rather than by increasing CPE specific binding or by directly activating the CPE molecule. Additional studies are required to further clarify the mechanism by which IFN sensitized Vero cells to CPE.

Primary action of Clostridium perfringens type A enterotoxin on HeLa and Vero cells in the absence of extracellular calcium: rapid and characteristic changes in membrane permeability

Clostridium perfringens type A enterotoxin bound rapidly to HeLa and Vero cells in the absence of extracellular Ca2+ at 37 degrees C. The bound toxin rapidly (within 2 min) caused influx of Na+ and efflux of K+ and Mg2+. Changes in membrane permeability occurred in the absence or presence of extracellular Ca2+ and to the similar extents at 37 degrees C and 4 degrees C, in contrast to the subsequent bleb and balloon formation, which required both extracellular Ca2+ and incubation at 37 degrees C. Substances with molecular weights of over ca. 200 protected the cells from the morphological alterations induced by the toxin, whereas substances with molecular weights of less than ca. 200 did not. The mechanism of the primary action of the enterotoxin is discussed.

Effects of Ca2+ and other cations on the action of Clostridium perfringens enterotoxin

We investigated the role of extracellular Ca2+ in the Clostridium perfringens enterotoxin-induced alteration of the permeability of the plasma membrane. Enterotoxin released 86Rb and 51Cr from the Vero cells preloaded with the isotope. In the presence of EGTA, however, it released 86Rb but not 51Cr. The binding of enterotoxin to the cells was not influenced by Ca2+ or Mg2+. The effects of various cations on the enterotoxin-induced 51Cr release was also studied. The release depended on extracellular Ca2+ but not on Mg2+; it was inhibited by each of Zn2+, La3+ and Co2+. Zn2+ and Co2+ also inhibited 51Cr release caused by the enterotoxin previously bound to the cell membrane. In contrast, antibody against enterotoxin did not neutralize the toxin once it was bound to the Vero cells. When the cells were treated with enterotoxin, 45Ca influx occurred and reached the plateau in a few minutes, as did 86Rb release.

Production and characterization of monoclonal antibodies to Clostridium perfringens enterotoxin

Four hybridoma cell lines producing monoclonal antibodies to Clostridium perfringens enterotoxin were established by fusion of mouse myeloma and spleen cells obtained from mice immunized with the enterotoxin and its toxoid. An enzyme-linked immunosorbent assay indicated that the two antibodies, 2-B-4 and 3-G-10, bound to those regions that were located close each other; the others, 3-B-2 and 2-H-2, bound to other independent regions on the enterotoxin. Release of 51Cr from Vero cells with the enterotoxin was inhibited by either 2-B-4 or 3-G-10, both of which inhibited the binding of 125I-labeled enterotoxin to the cells. Neither binding nor cytotoxicity of the enterotoxin was affected by 2-H-2; 3-B-2 only barely inhibited the binding but neutralized the enterotoxin shown by 51Cr release. It seems justified to conclude that 3-B-2 blocks the toxic action after the enterotoxin has bound to Vero cells.

Comparison of receptors for Clostridium perfringens type A and cholera enterotoxins in isolated rabbit intestinal brush border membranes

The rabbit intestinal brush border membrane (BBM) receptors for Clostridium perfringens type A (CPE) and cholera (CT) enterotoxins were compared. Initial studies characterized binding of 125I-CPE to isolated BBMs as specific, saturable, and irreversible. BBMs appear to contain a single type of CPE binding site. Protease pretreatment of BBMs strongly reduced subsequent specific binding of 125I-CPE but not 125I-CT, while neuraminidase pretreatment had little effect on binding of either enterotoxin. Proteases did not significantly release pre-bound 125I-CPE. Preincubation of CPE with an affinity-purified preparation containing a previously identified (Biochem. Biophys. Res. Commun. 112, 1099-105) CPE-binding protein resulted in reduced specific binding of 125I-CPE and an inhibition of CPE biologic activity. Similar experiments showed that CPE-binding protein had no effect on CT binding or biologic activity. Gangliosides had no significant effect on specific binding or biologic activity of CPE but reduced CT binding and biologic activity. Lipids, including gangliosides, separated by thin layer chromatography specifically bound CT but not CPE. Preincubation of BBMs with CT did not reduce subsequent binding of 125I-CPE; conversely, prebound CPE did not affect subsequent 125I-CT binding. These results strongly suggest that CPE does not share the CT BBM receptor ganglioside GM1, and support a role for the CPE-binding protein in CPE binding.

Morphological alterations and changes in cellular cations induced by Clostridium perfringens type A enterotoxin in tissue culture cells

The morphological alterations (bleb-balloon formation) induced by Clostridium perfringens type A enterotoxin in HeLa and Vero cells were studied under defined extracellular conditions. The action of enterotoxin was found to depend on the temperature but not on energy metabolism. The morphological alterations by the enterotoxin occurred in phosphate buffered saline containing Ca2+ and Mg2+. Of the constituents of the buffered saline, Ca2+ was essential for the morphological alterations and other ions were interchangeable. The morphological alterations by the enterotoxin occurred also in 10 mM Hepes-Na buffer, pH 7.2 containing NaCl, KCl or choline chloride at a concentration of over ca. 50 mM and in 10 mM Hepes-Ca buffer, pH 7.2 containing CaCl2 at a concentration of over ca. 50 mM. Addition of sucrose to the medium prevented induction of the morphological alterations. The amount of sucrose necessary to protect the cells increased with increase in NaCl, KCl or CaCl2 concentration in the medium. A calcium ionophore A23187 mimicked the action of enterotoxin. Examination of the cation contents of the cells by atomic absorption spectrophotometry showed early and rapid increase of Ca2+ during intoxication with concomitant changes in Na+, K+ and Mg2+ that reduced the ion concentration gradients between inside and outside of the cell present before toxin treatment. The mechanism of action of C. perfringens type A enterotoxin is discussed on the basis of these findings.

Production and characterization of monoclonal antibodies against Clostridium perfringens type A enterotoxin

Hybridomas secreting monoclonal antibodies (MABs) specific for Clostridium perfringens type A enterotoxin were produced by fusion of P3X63Ag8.653 myeloma cells with spleen cells from BALB/c mice immunized with purified enterotoxin. Wells containing hybridomas secreting immunoglobulin G (IgG) antibodies against enterotoxin were specifically identified by an indirect enzyme-linked immunosorbent assay (ELISA), and 10 ELISA-positive hybridomas were selected and cloned twice by limiting dilution. All 10 hybridomas produced MABs containing immunoglobulin G1 heavy chains and kappa (kappa) light chains. These hybridomas were then grown as ascitic tumors in mice, and MABs were purified from the ascites fluids with DEAE Affi-gel blue. The specificity of the MABs for enterotoxin was demonstrated by immunoblotting and ELISA. Competitive radioimmunoassay with 125I-MABs suggests that these MABs recognized at least four epitopes on the enterotoxin molecule. The enterotoxin-neutralizing ability of MABs from both hybridoma culture supernatants and ascites fluids was assessed by using a 3H-nucleotide-release Vero (African green monkey kidney) cell assay. Only 2 of the 10 hybridomas produced MABs which completely (greater than 90%) neutralized the biologic activity of enterotoxin. Preincubation of 125I-enterotoxin with MABs demonstrated that MAB neutralizing ability correlated with MAB-specific inhibition of specific binding of enterotoxin to intestinal brush border membranes.

Activation of the hole-forming toxin aerolysin by extracellular processing

A precursor-product relationship between aerolysin and a protein with a higher molecular weight was observed in culture supernatants of Aeromonas hydrophila. The larger protein was isolated by ammonium sulfate precipitation and ion-exchange and hydroxyapatite chromatography and compared with purified aerolysin. It was at least 250 times less hemolytic than aerolysin. Both proteins had the same amino acid sequence at the amino terminus. Cyanogen bromide fragments obtained from the two were identical except that each protein contained one unique fragment, and the fragment from the larger protein was 2,500 daltons larger than the fragment obtained from aerolysin. Treatment with trypsin or with an extracellular Aeromonas protease resulted in rapid conversion of the larger protein to a form corresponding in molecular weight and activity to aerolysin. The results indicate that aerolysin is exported to the culture supernatant as a protoxin which is later activated by proteolytic removal of a peptide from the C terminus.

Purification and properties of an enterotoxin from a coatless spore mutant of Clostridium perfringens type A

A method is described for isolating an enterotoxin from a coatless spore mutant (8-6) of Clostridium perfringens type A. The characteristics of this enterotoxin only slightly resembled those of previously isolated enterotoxins of C. perfringens. The type A (8-6) enterotoxin was found to be composed of two subunits of Mr 18 000 with isoelectric points of 3.8 and 4.3. The LD50 for mice was 39 micrograms/kg with 0.10 micrograms corresponding to one erythemal unit. The type A (8-6) enterotoxin was inactivated by heating for 10 min at 60 degrees C. The amino acid composition data of type A (8-6) and delta toxins was similar, but type A (8-6) and type A enterotoxins showed less similarity. This lack of similarity between type A and type A (8-6) enterotoxins was confirmed by the failure of anti-sera to type A enterotoxin to neutralize the type A (8-6) enterotoxin, in both the mouse and erythemal tests.

The effect of Ca++ and Mg++ on the action of Clostridium perfringens enterotoxin on Vero cells

Clostridium perfringens enterotoxin binds to receptors on Vero cells. This process does not depend on the presence of divalent cations (Ca++, Mg++). Binding of enterotoxin causes inhibition of 14C-leucine incorporation into proteins, probably because of depression of amino acid transport. The presence of Mg++ speeds up this effect of the enterotoxin. The enterotoxin produces membrane leakage only in the presence of Ca++, but additional Mg++ increases the rate of this process. These results indicate that the dissociation constant of the enterotoxin receptor interaction is reduced in the presence of Mg++. A model for the mode of action of the enterotoxin is proposed.

Osmotic stabilizers differentially inhibit permeability alterations induced in Vero cells by Clostridium perfringens enterotoxin

Using a sensitive Vero (African green monkey kidney) cell model system, studies were performed to further investigate whether Clostridium perfringens enterotoxin acts via disruption of the colloid-osmotic equilibrium of sensitive cells. Enterotoxin was shown to cause a rapid loss of intracellular 86Rb+ (Mr approx. 100) with time- and dose-dependent kinetics. The enterotoxin-induced release of intracellular 86Rb+ preceded the loss of two larger labels, 51Cr label (Mr approx. 3500) and 3H-labeled nucleotides (Mr less than 1000). The osmotic stabilizers, sucrose and poly(ethylene glycol), differentially inhibited enterotoxin-induced larger label loss versus 86Rb+ loss. Further, enterotoxin was shown to cause a rapid influx of 24Na+ that was not significantly inhibited by osmotic stabilizers. Additional studies demonstrated that lysosomotropic agents were not protective against characteristic enterotoxin-induced membrane permeability alterations or morphological damage. Taken collectively, these results are consistent with an action for enterotoxin which involves a disruption of the osmotic equilibrium.

Assessment of membrane damage in continuous cultures of mammalian cells

The present studies were aimed at evaluating procedures for assessing the effect of chemicals on the integrity of the plasma membrane in continuous cell cultures. The degree of membrane damage was monitored by determining the 'leakage' of alpha-[3H]aminoisobutyric acid ([3H]AIB) and [14C]deoxy-2-fluoro-D-glucose ([14C]FdG) from the prelabelled cells. These parameters were compared to the loss of lactate dehydrogenase (LDH) from the cells and the decrease in the intracellular level of K+. Triton X-100, sodium dodecylsulfate (SDS), phospholipase C and nystatin which are known to affect membranes by different mechanisms served as test agents. In parallel, we monitored the effects of the chemicals on the viability of the cells. The following results were obtained: (1) The two radioactive markers [3H]AIB and [14C]FdG were found to be suitable to probe for damages of the plasma membrane in a variety of continuous cell lines which differ widely in their phenotype, rate of growth and degree of differentiation. (2) The leakage of the two markers could conveniently be monitored by double labelling techniques. (3) The loss from the cells of the 3 markers of smaller molecular size, K+, [3H]AIB, [14C]FdG, differed considerably depending on the test agent used. (4) Intracellular K+ level and [3H]AIB leakage generally appeared to follow a similar pattern, whereas [14C]FdG leakage may have shown a distinctly different response. (5) The leakage of LDH was an insensitive indicator for membrane damage. (6) No clear relationship was detectable between a particular leakage pattern of the markers and the loss of cellular viability.

Sequence of the amino-terminal part of enterotoxin from Clostridium perfringens type A: identification of points of trypsin activation

The sequence of the first 66 amino acids of the amino-terminal part of the enterotoxin from Clostridium perfringens type A is presented. The trypsin activation of the enterotoxin involves hydrolysis of Lys15-Glu16 and Lys25-Thr26 bonds. The N-terminal sequence of the trypsin-activated enterotoxin has limited homology with the sequence of the N-terminal region of the cholera toxin B subunit.

Inhibition of protein synthesis by a tryptic polypeptide of Clostridium perfringens type A enterotoxin

The biological activity of Clostridium perfringens enterotoxin can be tested more precisely and with a much higher sensitivity by using the inhibition of protein synthesis by Vero cells, rather than the guinea pig skin test. Tryptic peptides of the enterotoxin produced in the presence of different concentrations of sodium dodecyl sulfate (0-1%) have been tested for biological activity (Vero cells) and inhibitory effect on cell-free protein synthesis (rabbit reticulocyte lysate). A fraction of tryptic peptides, about 16,000 daltons, was able to inhibit the cell-free protein synthesis, while the native enterotoxin had no such effect. The 16 kDa fraction had, however, lost the ability to disrupt the Vero cells (normal biological activity). It is probable that the enterotoxin has the double function (A and B chain), known from several other toxins, confined in its single polypeptide chain.

Clostridium perfringens type A enterotoxin: characterization of the amino-terminal region

The amino-terminal region of the enterotoxin of Clostridium perfringens was investigated by automated sequence analysis. The primary structure results revealed that the enterotoxin is composed of a single polypeptide amino acid sequence. Computer comparison of a 20-residue sequence with a sequence library of reported proteins revealed no significant chemical similarities, indicating that the enterotoxin represents a unique polypeptide primary structure.

Effects of temperature, pH and detergents on the molecular conformation of the enterotoxin of Clostridium perfringens

The effects of temperature, pH and sodium dodecyl sulfate on the conformation of the enterotoxin from Clostridium perfringens type A were followed by circular dichroism in both the peptide and aromatic regions. At near-physiological conditions (35 degrees C, pH 6.7) the enterotoxin exhibited a conformation consisting of approximately 60% pleated sheet, 40% non-periodic, and essentially no helix. The peptide region was relatively stable at temperatures up to 55 degrees C and at pH values ranging from 4-10. The aromatic region demonstrated profound, time-dependent changes at 55 degrees C. At temperatures greater than 55 degrees C, extremes of pH, and in the presence of SDS, the spectra in both regions showed major structural reorganization; in most cases a gain in helical content at the expense of sheet structure was observed. The conformational properties of the protein are very similar to those observed for the lectins, a group of carbohydrate-binding proteins.

Protective effects of osmotic stabilizers on morphological and permeability alterations induced in Vero cells by Clostridium perfringens enterotoxin

Culture medium made hypertonic by the addition of osmotic stabilizers such as sucrose, poly(ethylene glycol), dextran and bovine serum albumin protected against changes in morphology and plasma membrane permeability induced by Clostridium perfringes enterotoxin. The protection did not appear to be due to binding inhibition. Results of these studies support an osmotic disruption mechanism for the action of the enterotoxin. A comprehensive model of the enterotoxin's action based on an osmotic disruption mechanism is proposed.